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Kinetics and Mechanism of Photoconversion of N-Substituted Amides of Salicylic Acid
Studied using optical spectroscopy, stationary, and nanosecond laser photolysis (Nd:YAG laser 355 nm) conversion products in heptane of N-substituted amides of salicylic acid: N-(4-hydroxyhydro-3,5-di-tert-butylphenyl) amide 2-hydroxy-3-t-butyl-5-ethylbenzoic acid (1), 2-(4-hydroxy-3,5-hydroxybenzoic acid)-di-tert-butylphenyl) propyl] amide (2), N-(4-hydroxyphenyl) amide 2-hidroxy benzoic acid (3), and 2-hydroxy-3-t-butyl-5-ethylbenzoic acid N-[3-(4-hydroxy-3,5-di-t-butylphenyl) propyl] amide (4). It is shown that amides exist both in the unbound state and in complexes with intra- and intermolecular hydrogen bonding. Free phenolic groups of amides undergo photolysis, which leads to the formation of a triplet state and phenoxyl radicals RO∙, presumably due to the absorption of the second photon by the excited singlet state. Triplet-triplet annihilation and recombination (kr ≈ 2.3·108 L mol−1 s) are the main channels for the decay of the triplet state and radicals RO∙. UV irradiation of the compounds leads to the excitation of amide groups, and radical products are not formed due to ionization of the NH bond. The process of initiated UV oxidation of the model substrate (methyloleate) in the presence of amides 1–4 was compared with the known antioxidants (AO): dibunol (2,6-di-tert-butyl-4-methylphenol) (5) and α-tocopherol (6-hydroxy-2,5,7,8-tetramethyl-2-phytyl chromane) (6). It has been shown that all amides of salicylic acid (I–IV) effectively inhibit the oxidation of methyl oleate, initiated by UV irradiation. The mechanism of the inhibitory effect of compounds has been established, which is associated with the possibility of direct interaction of phenols with free radicals (antiradical activity). Testing of antiradical activity of amides (I–IV), estimated by the method of chemiluminescence, made it possible to determine the range of reaction rate constants with peroxyl radicals RO∙2 k7 = (0.52–6.86) • 104 m L−1 s−1. The interrelation of antioxidant properties of amides of salicylic acid (I–IV) and features of their structure is established. It was shown that the introduction of o-tert-butyl substituents and the separation of aromatic fragments by three methylene groups lead to a significant increase in antioxidant activity.
Part of the book: Photochemistry and Photophysics